Aspen trees and conifers grow together on the Colorado landscape. Aspens grow in open areas that have been disturbed, or where other trees have disappeared. (Special to the Reporter-Herald/Mario Bretfeld)

During winter, conifer trees -- such as spruce, fir and pine -- decorate the season, displaying newly fallen snow on their green branches. But it's the aspen tree, Colorado's main native deciduous (or leaf-dropping) tree that brings identity to the state. Visitors and locals alike admire the quaking leaves, and the "gold rush" of the fall as leaves turn a golden yellow.

In Greeley, University of Northern Colorado doctoral candidate Mario Bretfeld of Germany became fascinated with aspens while doing a study at Yellowstone National Park a few years ago. He was involved in research to see how the introduction of wolves had affected elk behavior in the park, including the aspen browsing habits of elk.

"Aspen are a very valuable food source for elk, deer, and ungulates in general," says Bretfeld. Additionally, aspen tend to create more humus-rich soil underneath. While conifers shed needles each year that inhibit growth on the forest floor, aspen leaves degenerate quickly, he explains. This helps create nutrient-rich soil with a lot more understory vegetation, beneficial to both plants and animals.

In Colorado, he is focusing on how the pine beetle epidemic affects aspen groves. Specifically, Bretfeld wonders whether the beetle outbreak actually has a positive effect for aspen growth, particularly in areas where aspens seem to be declining.

Aspens thrive after some sort of disturbance such as fire, clear-cutting of a forest, disease or insect infestation. Conifer trees grow in moist mountain climates, increasing in size and breadth until a disturbance occurs. When the trees disappear, the aspen roots which sometimes have been lying underground dormant for years, spring forth with new shoots to regrow the forest.

Mario Bretfeld, a doctoral candidate at the University of Colorado in Greeley, measured the flow of water and sap between aspen trees in Fraser to find out how aspens share resources within a grove.
(Special to the Reporter-Herald/Mario Bretfeld)

Aspens also help prevent erosion by retaining water runoff, Bretfeld continues. "Due to their connected nature of roots underneath, they are really good in preventing landslides." Another benefit is the firebreak that aspen groves create. "They don't burn as easily as conifers do. There is no needle layer underneath that burns away and there is more moisture."

What Bretfeld expects to find is that aspen groves are persisting well, or perhaps increasing in areas where the landscape has lost pine trees due to the mountain pine beetle. "It is important to mention that pine beetles are a natural part of the ecosystem," says Bretfeld. "They've always been here." Climate changes and fire suppression are two of the factors he mentions that have possibly given pine beetles a chance to emerge in great numbers.

Bretfeld was fortunate to be able to refer to and use a vegetation diversity study performed along the Front Range in the 1970s by Dr. Robert K. Peet (now at the University of North Carolina). Bretfeld and University of Northern Colorado professor Scott B. Franklin surveyed 305 of Peet's plots, of which 95 had aspen recorded. Most of those 95 still have some evidence of aspen in some form. This comparison has allowed him to see how aspen groves have changed, and many cases persisted.

While aspen trees produce seeds, most growth happens with shoots directly from the roots of an existing tree. "Most aspen groves you see are comprised of multiple clones that have grown together," shares Bretfeld.

To learn more about how aspen share resources within a clone, Bretfeld did another study. Using two aspens connected by roots, he cut all the roots off one tree, except for one root that tied it to the neighboring tree. Bretfeld had measured the flow of water throughout the roots before the cut. Following the root amputation, the fully rooted tree began to increase its flow of water to the rootless neighbor.

"You can think about it as a nursing effect -- the big tree is trying to help the guy who lost all the roots," says Bretfeld, adding that it could also be a parasitic reaction of the rootless tree, pulling at water resources wherever it can. Regardless, it demonstrates a survival technique that benefits trees that must survive in an arid region.

Fire and disturbance are part of the cycle of regeneration and regrowth that enable aspens to grow. "Without the cycling, we wouldn't have aspen here anymore," says Bretfeld. "They need the fresh, blank slate to actually re-sprout and go crazy again and grow."

While a human lifetime is short in comparison to the life of a forest, Bretfeld offers a reminder that "Even though everything is brown, it will be green again." Whether it be fire or pine beetles, the forest cycle continues.